This invention relates generally to the dimerization of alpha-olefins and more specifically to a process for the dimerization of alpha-olefins to vinylidene olefins using catalyst compositions containing a metallocene such as bis(cyclopentadienyl)-zirconium dichloride, an alkylaluminoxane other than methylaluminoxane such as tri-isobutylaluminoxane, and trimethyl aluminum.
Olefin dimerization using catalysts which contain methylaluminoxane (MAO) in combination with a transition metal metallocene is disclosed, for example, in U.S. Pat. No. 4,658,078. These catalysts provide high polymerization activity but the synthesis of the MAO component is difficult and expensive because of the high reactivity of trimethylaluminum with water and the relatively low yields compared to alkylaluminoxanes such as tri-isobutylaluminoxane. Although easier to prepare, such higher alkylaluminoxanes provide inferior results when used as cocatalysts with the metallocenes. I have now discovered that catalyst systems which employ a combination of higher alkylaluminoxane and trimethylaluminum in place of methylaluminoxane provide excellent yields of high purity vinylidene olefins by the dimerization of alpha-olefins. Such vinylidene olefins are useful intermediates in preparing a number of products such as specialty detergents and lubricant additives.